HUMIDITY-REGULATED AND RECIRCULATING DRY KILN. 15 
wood, the rate depends largely upon the relative humidity. 1 There 
is a balance between what might be termed the retentive or attractive 
property of the wood, " hygroscopicity," and the tendency of the 
moisture to vaporize. It is the difference between the tension of the 
vapor at the higher temperature of the wood and the tension actually 
existing in the space surrounding the wood. This retentive prop- 
erty increases as the wood becomes drier and decreases as it ap- 
proaches the wet condition. Experiments indicate that generally 
it is nearly inversely proportional to the amount of moisture remain- 
ing in the wood. 
EVAPORATION WHEN AIR IS PRESENT. 
When air is present with the superheated steam or water vapor 
the conditions are quite different. Vaporization of a particle from 
the surface of the free water is retarded by the air pressure, so that 
the temperature of the water may be raised above the dew point. 2 
The air now, as well as the vapor, conducts heat to the water, so that 
the rate of evaporation at given pressures depends not alone on the 
quantity of heat supplied (by circulation and degree of superheat- 
ing) but upon the relative amounts of vapor and air present. That 
is to say, the lower the relative humidity the greater is the rate of 
evaporation at a given temperature and pressure. The temperature 
of the water will correspond to that of the wet bulb, and not to that 
of the dew point. When the wood becomes partially dried its tem- 
perature will rise, as in the case of superheated steam, and it may 
be heated even above the boiling point at the given pressure without 
giving up all of its moisture, provided there is some vapor in the air. 
CONCLUSIONS AS TO DRYING IN VAPOR ALONE AND IN AIR AND VAPOR. 
Thus it is seen that the rate of drying may be controlled by the 
relative humidity, provided there be sufficient circulation to supply 
the heat required. In the case of steam alone, the rate of drying, as 
just shown, depends upon the quantity of circulation as well as the 
degree of superheating. Hence the conclusion follows that moist 
air, with ample circulation, should give more uniform drying 
throughout than superheated steam, which varies with the rate of 
circulation in each portion. 
1 In using the term relative humidity as applied to superheated steam it is understood 
to mean the ratio of the actual vapor pressure to that of the pressure of saturated vapor 
at the given temperature, as explained before. 
2 In reality what probably happens is that the layer of air in immediate contact with 
the water becomes saturated and has a higher vapor pressure corresponding to the tem- 
perature of the surface of the water, and the air retards the diffusion of this vapor. The 
temperature of the water, however, can not exceed the boiling point for the given pres- 
sure, at which point the conditions must become the same as those for superheated steam 
alone, since then the air will become entirely displaced by the water vapor. 
